This invention relates generally to magnetic resonance imaging (MRI), and more particularly, to transmit coil arrays used in MRI.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person's body that is to undergo MRI. The homogeneous magnetic field conditions the interior of the person's body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more coils placed on or about the person.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the nucleus' location. As is well known in the art, it is possible to distinguish radiation originating from different locations within the person's body by applying a field gradient to the magnetic field across the person's body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Radiation of a particular frequency may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person's body. The application of such a field gradient is also referred to as frequency encoding.
However, the application of a field gradient does not allow for two-dimensional resolution, since all nuclei at a given left-to-right position experience the same field strength, and hence emit radiation of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern radiation originating from the top versus radiation originating from the bottom of the person at a given left-to-right position. Resolution has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction to thereby perturb the nuclei in varied amounts. The application of such additional gradients is also referred to as phase encoding.
Frequency-encoded data sensed by the coils during a phase encoding step is stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated by using various imaging applications where a Fourier transformation of the k-space matrix is performed to convert frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
In many imaging applications, examination of the object with spatial-spectral selectivity (that is, imaging a particular spectral component in a particular region-of-interest) is desired in order to meet both clinical needs (such as water/fat imaging for examining atherosclerotic plaques and reduced-FOV imaging for accelerating scans) as well as quality requirements (for example, reduction of image artifacts due to frequency shifts in SSFP and fast GRE sequences).
To induce spatial-spectral selectivity, nuclear magnetic resonance (NMR) excitation that induces transverse magnetization to all spins of a prescribed Larmor frequency range in a prescribed region of interest can be used. However, with existing methods where a volume transmit coil that effects a relatively uniform RF field (e.g., a birdcage coil with quadrature driving) is used for RF transmission, an NMR excitation that achieves spatial-spectral selectivity often involves intensified pulsing activity, which require powerful gradients to keep pulse duration in check. On clinical scanners limitations in gradient strength or switching rate usually render impractical the use of spatial-spectral pulses that are selective along multiple spatial dimensions.
What is needed is a method and system to enable acceleration of multi-dimensional spatial spectral selective pulses during excitation.